1. Field of the Invention
The invention pertains to the field of signal detectors and more specifically to signal detectors having mechanisms to achieve automatic compensation for variations in ambient conditions such as temperature and pressure.
2. Description of the Prior Art
Photoelastic materials have been employed in highly rugged, sensitive pressure sensors such as described in U.S. patent application Ser. No. 248,616 filed by McMahon on Mar. 27, 1981, now abandoned and assigned to the assignee of the present invention. Sensors of this type are constructed to transfer external pressures as a uniaxial force to a channel in a photoelastic medium such that optical birefringence is produced and sensed by a collimated light beam that propagates therethrough. To sense optical birefringence the polarization of the input light beam may be resolved into orthogonal components respectively parallel to the optical axes of the photoelastic medium and the phase interference between these two beams measured. The resulting signal is a function of the birefringence induced by the pressure applied to the photoelastic material. In a structure of this type light is coupled to the sensor through an input fiber, it is then collimated, polarized at an 45.degree. angle relative to the stress axis of the photoelastic material, coupled to a channel therein, split into respective paths for each polarization component and the beams in each path thereafter refocused to exit via respective output fiber. Additionally, a quarter wave plate may be positioned between the input polarizer and the propagating channel to optically bias the orthogonal polarizations so that the sensor responds linearly to small external pressure variations.
This principle may be employed to construct photoelastic hydrophone by bonding the photoelastic channel between a metal support and metal diaphragm. The metal diaphragm intercepts the force equal to the pressure times the diaphragm area and applies this force to the smaller critical area of the photoelastic channel, while the support beam provides an immobile backing which insures that the bulk of the elastic yield is in the photoelastic channel. The assembly may be positioned in a fluid filled chamber which is sealed by the metal diaphragm at one end and coupled to a fluid reservoir at the other. As the ambient pressure and temperature change, fluid transfer between the chamber and the reservoir takes place. This transfer reduces, but does not eliminate, the stresses within the photoelastic channel. Thus, the optical bias may vary and adversely effect the hydrophone operation.
Hydrophones are normally expected to operate over a temperature range of 0.degree.-40.degree. C. and a hydrostatic pressure range 0-1,000 PSI. Since Young's modulus and the temperature coefficient of expansion of the metal are not the same as that of the photoelastic material, it is apparent that hydrostatic pressure and ambient temperature changes cause the metal and photoelastic material to yield or elongate by different amounts. This resulting difference in elongation at a joined surface causes substantial stresses to build up in the photoelastic material, that not only move the hydrophone away from its maximum sensitivity bias point, but may cause it to approach or pass through a zero sensitivity bias condition.
To operate efficiently, hydrophones must couple acoustic pressures to the photoelastic material with a minimal of attenuation, and maintain the desired optical bias by preventing stresses therein caused by the bonding of dissimilar materials that behave differently under changes of ambient pressure and temperature. One solution to the problem is to make the diaphragm, channel, and support from the same photoelastic material so that equivalent fractional dimensional changes occur for all portions of a composite structure when subjected to temperature and pressure changes. Disadvantages of this approach are two-fold;
First, the photoelastic support adjacent to the channel is much thicker than the channel and can elongate under acoustic pressures with a dimensional change that is comparable to the channel dimensions. The equal elongations of the channel and support, forces the diaphragm to move twice as far to transmit the same pressure as it would have moved were only the photoelastic channel to yield. Doubling the diaphragm motion requires the volume of the fluid filled chamber to double, thereby effectively doubling the size of the hydrophone. Under an applied stress, the support and photoelastic material function in series to provide part of the restoring force that limits the diaphragm motion. The remainder of the restoring force is supplied by the elasticity of the fluid in the chamber. If the chamber is too small the "spring constant" of the fluid may be comparable or greater than the "spring constant" of the photoelastic channel plus its support. Under these conditions, only a fraction of the total acoustic force is applied to the photoelastic material. Consequently, a sufficiently large chamber must be used if substantially all the acoustic pressure is to be applied to the photoelastic material.
Second, fabricating the diaphragm, channel, and support from the same photoelastic material produces a relatively large and complex shaped structure, permitting significant temperature differences to exist therethrough. These temperature differences may produce strains in the photoelastic channel that move the optical bias from the maximum sensitivity condition. Thus, while an all photoelastic construction substantially removes sensitivity to static or slowly varying temperature changes, the hydrophone may remain intolerably sensitive to more rapid fluctuations in external temperature.